Terán was appointed governor on January 23, 1691, by Viceroy Gaspar de la Cerda, 8th Count of Galve.
His role as governor was to set up seven missions among the Tejas Indians; to seek and remove any foreigners that may have settled in Spanish territory; and to catalog the land, the natural resources, and the peoples of the area.
Terán and his company departed on their trek on May 16, 1691, from Monclova, crossed the Rio Grande on May 28, and had reached the Red River by December of that year.
[2] On his journey northward, Terán met with Gregorio de Salinas Varona at the site of the French Fort Saint Louis, which had been abandoned after colonists died from disease and a Karankawa Indian attack.
When Terán was travelling southward, he met Juan Enríquez Barroto at Matagorda Bay on March 5, 1692, who relayed orders from the viceroy to explore the lower Mississippi River area.